NCHU System & Network LabNCHU System & Network Lab

Lab 15Lab 15

Record Locking

Record Locking (1/4)Record Locking (1/4)

• What happens when two process attempt to edit the same file at the same time ?

• Record locking is the ability of a process to prevent other processes from modifying or reading a region of a file while the first process is working on it.

Record Locking (2/4)Record Locking (2/4)

• Here are two types of locks :– Read lock (shared lock)

• Any number of processes can have a shared read lock on a given region of a file.

• All of them can read from the file but can’t write to it.

– Write lock (exclusive lock)• Only one process can have an exclusive lock on a given

region of a file.

• The other process are prevented from getting locks or reading and writing.

Record locking (3/4)Record locking (3/4)

• Advisory V.S. Mandatory locking– Advisory lock (cooperative lock)

• All access functions in a library handle record locking in a consistent way.

• But advisory locking doesn’t prevent some other process that has write permission for file from writing.

database

Advisorylock

accesslibrary

A rude process with write permission of the file

Record locking (4/4)Record locking (4/4)

– Mandatory locking cause the kernel to check every read(), write(), open() to prevent the calling process from violating the lock rule.

file kernel

fget()

write()

readn()

fput()

Locking check

fcntl()fcntl() (1/3) (1/3)• fcntl() function can change the properties of a file

that is already open.– The fcntl() is used for five different purposes :

cmd description

F_DUPFD Duplicate an existing descriptor

F_GETFD,F_SETFD Get/set file descriptor flags

F_GETFL,F_SETFL Get/set file status flags (O_RDWR … etc)

F_GETOWN,F_SETOWN Get/set asynchronous I/O ownership

F_GETLK,F_SETLK,F_SETLKW

Get/set/lock file lock, it discussed later.

#include <fcntl.h>int fcntl (int filedes, int cmd, struct flock *lockptr…);

fcntl()fcntl() (2/3) (2/3)

• For record locking ,the third argument of fcntl() is a pointer to a flock structure :

struct flock {short l_type; /* F_RDLCK, F_WRLCK, F_UNLCK */off_t l_start; /* offset in bytes, relative to l_whence */short l_whence; /* SEEK_SET, SEEK_CUR, SEEK_END */off_t l_len; /* length, in bytes ; 0 means lock to EOF */pid_t l_pid; /* returned with F_GETLK */

}

fcntl()fcntl() (3/3) (3/3)

• Three commands of fcntl( record lock ) :– F_GETLK

• To check the lock described by flockptr :– The information pointed by flockptr is overwritten by an existing lock

.

– If no lock exists, flockptr.l_type = F_UNLCK

– F_SETLK• Set the lock described by flockptr.

• If we try to obtain an illegal record lock, fcntl() will return errono = EAGAIN.

– F_SETLKW• A blocking version of F_SETLK. It will wait until the desired regio

n released.

DeadlockDeadlock

• Deadlock– Deadlock occurs when two processes are each

waiting for a resource that the other has locked.

– When a deadlock is detected , the kernel must choose one to receive the error return.

• It depends on different implementations of systems.

Process A

Process B

Lock request

Lock request

Inheritance and Release of LocksInheritance and Release of Locks

• Inheritance and release of locks :– Release of locks :

• When a process terminates, all its locks are released.

• When a descriptor is closed, any locks on it are automatically released.

– Inheritance• Locks are never inherited by the child process across a fork().

• Locks are inherited by a new program across an exec() .

Ex : FreeBSD file lock structureEx : FreeBSD file lock structure

Figure : FreeBSD data structures for record locking

Lab 1Lab 1

• Write a program that :– User can define a lock with type, start, and length.– This program will try to get a lock or it will be

blocked by an existing lock.• If it is blocked , it should show the lock information.

– By running this program in two or more windows, you can see how programs interact while waiting locks.

Lab 1 (cont.)Lab 1 (cont.)

Input : R/W, start, length

Is it locked?

Show the lock and wait..

yes

Lock the region

unlock

Wait for user

No

1

23

4

5

#include <stdio.h>#include <stdlib.h> #include <errno.h> #include <fcntl.h> #include <unistd.h>

int main() {struct flock fl = { F_WRLCK, SEEK_SET, 0, 0, 0 }; int fd; fd = open(“test", O_RDWR)fcntl (fd, F_SETLKW, &fl) ; // set the lock ‘fl’

getchar();printf(“file is locked…”);

…exit(1);

}

Lab 2Lab 2

• Use the program you have done before :– You try to make a deadlock situation to see that

how does Linux solve this problem.

ReferenceReference•Advanced Programming in the UNIX Environment 2nd

Author : Richard Stevens, Stephen A.Rago, Publisher : Addison-Wesley

•Beginning Linux ProgrammingAuthor : Richard Stones, Neil Matthew Publisher : Wrox

•http://linux.vbird.org/•http://www.jollen.org/blog/ jollen’s Blog